1. Field of the Invention
The invention relates to new 4-substituted azetidinones having the formulae I and II: ##STR2## with R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and X defined hereafter, as intermediates for the preparation of carbapenem and carbacephem antibacterials through the utilization of an acid mediated ring closure reaction shown to form carbapenems III and carbacephems IV with R.sup.1 -R.sup.5 defined hereafter. ##STR3##